Electromagnetic radiation filters are well known and have many practical uses. One such use is in filtering laser light in order to protect individuals from its effects. As is well known, when the eye is exposed to laser light, significant damage can occur. This is mainly due to the absorption of incoming photons and the heating of the living eye tissue. The potential damage depends on the intensity of the laser, that is the energy per second from the laser impacting on a given surface area, and the duration of the exposure. Two known types of laser are continuous wave (CW) and pulsed lasers. CW lasers are classified based on their power output whereas pulsed lasers are classified based on their total energy per pulse.
Protection from laser light has become increasingly important in everyday practical situations due to the proliferation of cheaper and more powerful laser systems. Certain laser products have in recent years become cheap to manufacture and thus commercially readily available. These products include so-called “laser pens” or “laser light pointers”. In the UK there are regulations in place such that it is only allowable to manufacture and sell laser light pointers having a power of up to 5 milliWatts (mW). However these regulations are not consistent worldwide and it is possible in some countries to purchase cheaply a 1 Watt laser light pointer which has an effective area of up to four times the effective area of the most powerful laser light pointer available in the UK, and which can cause cornea damage to the human eye from as much as three miles away. Unfortunately there has been an increase in incidents of laser light pointers being deliberately pointed at vehicle drivers and also at aircraft whilst in flight. These incidents have potentially extremely serious safety consequences. Although laser light pointers do not have the ability to damage or destroy most vehicles or aircraft, they can and often do interfere with the ability of the driver, pilot or crew to maintain sufficient visual contact with the road, flight path or runway.
The potential safety consequences are particularly grave when a laser light pointer is directed at an aircraft during take off or landing.
Aircraft need to be at a relatively low altitude in order to be affected by the use of laser pointers on the ground. For this reason aircraft are most vulnerable during the approach phase of landing. During this phase an aircraft will typically be flying at around 6,000 feet and be lined up with the runway descending at a relatively steady rate of around 700 feet per minute. This makes the aircraft an object easy to aim at with a laser pointer. At the same time the crew onboard the aircraft will be increasingly focused on external cues outside the aircraft which allow the crew to control the speed, rate of descend and heading of the aircraft towards a successful landing on the runway. This makes the crew more prone to being affected by a laser beam pointer and potentially even to receive serious eye injuries. This is certainly the case when the crew is conducting a “visual landing” or a “non-precision landing”. During both such landing phases the flight crew navigate using primarily external cues to complete the approach and landing.
If a laser light is pointed at the crew at any stage of the approach phase the crew might be injured, lose momentarily sight of the runway or decide that a safe interference-free landing is not possible. This may lead to a “go-around” or “abort” in order to avoid an event which may significantly endanger the aircraft and the safety of those onboard. The go-around and its related procedures may lead to increased work load for flight crew and Air Traffic Control, which in turn may introduce other threats to safety. In busy airports such as those found in Europe, a go-around procedure might result in a high workload and relatively hazardous situation.
Pointing portable lasers at aircraft can also have an impact on the efficiency and cost of airline operations. A go-around involves the aircraft spooling the engines to a thrust setting of a take-off while climbing and then returning to the initial approach point to attempt another landing, all of which may last anywhere between 10 and 20 minutes. During such a manoeuvre, a 747-400 aircraft might burn up to 4 tonnes of additional fuel which at current prices may amount to around 6,000 USD. Other factors such as missed passenger connections and aircraft utilisation may make a go-around even more expensive.
There are many safety regulations and systems already in place for aircraft. Unfortunately some systems and procedures introduced to improve safety may actually increase the potential severity of laser pointers being aimed at aircraft. For example the use of Head-Up-Display systems (HUD), an expensive technology once used only in military aircraft, is finding its way increasingly into everyday commercial aircraft operations. The system is comprised of a glass in front of the pilot on which flight parameters and the position of the aircraft in relation to the runway are displayed. This system allows the flight crew to observe external cues as well as the aircraft attitude and speed (among other parameters) without having to look down to the instrument panel. This results in flight crew looking towards and out of the windshield throughout the approach and landing phase. Thus any laser point device aimed at the aircraft windshield will in all probability result in adverse effects to the crew.
There are numerous known solutions for filtering out laser light to protect user safety. For example Laser Protection Systems (LPS) are routinely used in laboratories around the world. They typically come in the form of goggles, eye-shields or contact lenses which are worn by the person susceptible to the laser radiation. They also come in the form of windows, which are placed around the laser location to protect the surroundings. These filters are usually built using polymers for low intensity lasers or glass for high heat densities.
There are several disadvantages associated with currently available LPS. They usually operate over a single band of light, providing protection from a single type of laser only. Additionally they are not sufficiently narrowband, thus they block more light than necessary and so distort the user's overall vision. LPSs are also usually tinted, artificially colouring the field of view. They therefore cannot always be used—for example, it is unsafe for an airline pilot to wear red goggles while flying an aircraft at night. And glass-based filters are heavy and cannot be comfortably worn by people.
No known system can provide filtration of electromagnetic radiation which is sufficiently accurate and focused for many practical purposes without distorting the propagation of electromagnetic radiation at other wavelengths that the user does not wish to filter. Furthermore many existing filters are impractical and/or too expensive for widespread use.
Aspects of an invention are set out in the appended independent claims.
There is provided a filter for selectively filtering electromagnetic radiation. The filter comprises a first metamaterial and a second metamaterial. Each metamaterial comprises a plurality of structural features having a size less than a predetermined wavelength. Electromagnetic radiation at the predetermined wavelength is blocked by the metamaterial owing to the carefully chosen structural features. The structural feature may be a thickness of a dielectric layer. The metamaterial may comprise a plurality of material elements and the structural feature may be the size of the material elements. The material elements may comprise any of: a metallic shape, a photonic crystal, a polymer material element or a liquid crystal. The metamaterials may comprise a nanostructured material, made from nanoscale material elements. The filter may provide optical transparency at all frequencies except at the selected frequency or frequencies which it is configured to block. Therefore it does not distort user vision except at the frequencies that have been deliberately blocked, for example particular laser frequencies that could cause harm to the user. The filter may block a single narrow frequency band or it may block a plurality of distinct narrow frequency bands. By combining metamaterials. the filter may block a selected frequency or selected frequencies of radiation over a range of angles.